The present invention relates generally to lighting sources and more particularly to lighting sources employing fluorescent bulbs as a primary illumination source.
Items that utilize backlighting, such as certain medical equipment (X-ray viewing lamps), projection screens and items incorporating Liquid Crystal Displays (LCDS) all require efficient and effective lighting sources. Typically, in such applications, as little as five percent of the emitted light is transmitted to the viewer. Fluorescent lighting sources have received widespread use due primarily to high brightness levels and low heat output characteristics.
Unfortunately, control of the fluorescent light source has proven a major drawback with regards to such use. A fluorescent bulb is very dynamic in nature, presenting the drive circuit a varying impedance at different brightness levels. The impedance determines the input power into the fluorescent bulb and accordingly the output. To complicate matters more, the fluorescent bulb is very temperature sensitive. Light output changes dramatically with temperature variations. These variables have been accommodated in the past, with limited success, by utilizing optical feedback techniques.
Optical feedback generally requires use of a photo sensor on or near the light source, thereby allowing the user to select a light reference level, enabling electronic circuits to control the light output level without further user intervention. This "solution" has presented additional challenges. The fluorescent bulb, during medium to low brightness levels, has bright and dim areas of emissions known as striations. These striations produce a visual flicker in the fluorescent light source when combined with optical feedback. The flicker is normally present at five to one hundred and twenty hertz and is easily detected by display users.
Electrical or time integration has been used to attack this problem, however it has several major drawbacks. First, time integration slows the response of the feedback control circuit, thereby making the backlight control circuit non-responsive to short duration changes in light output. The result is often a slow "pulsing" or "walking" flicker. Second, the backlight control circuit is slow to respond to changes in the light reference level, thereby making adjustment of the light output level difficult.
Spatial optical integration addresses the shortcomings of time integration by averaging the instantaneous light output over the length of the fluorescent bulb. By integrating the emitted light over the length of the tube, the striations are averaged out and the optical feedback "sees" a continuous light level. This technique eliminates flicker, yet allows the feedback circuit to respond instantaneously to variables that impact the overall light output.
Spatial optical integration can be accomplished in a variety of ways. Multiple photodetectors may be used to sense different areas of the fluorescent bulb. The outputs of all the photodetectors can then be summed to get an average light level. This technique is both costly and of limited use, since the value of the "dark" current of the photodetectors is also summed, thereby rendering the feedback circuit ineffective at low brightness levels when it is needed most.
Spatial sampling may also be utilized to perform optical integration. This method utilizes optical fibers to transmit light signals to a single photodetector. This method solves the usability problem at low light levels and replaces it with a costly production problem. Assembling twenty to fifty optical fibers from various points along the fluorescent bulb into a bundle at the photodetector requires special skills and equipment. Furthermore, two to four inches of the instrument's depth are required for the bend radius of the optical fibers.
Accordingly, a need exists for an improved apparatus for controlling the emitted light in particular applications of fluorescent light sources.